By: Maizura Daud
Nur Atiqah Hasbullah
Nor Husna Mohd Rosli
DC ELECTRICITY
Originated from Greek word – ‘amber’ . Modern word – ‘electron’
Du FayDiscover that there are two types of charge. Like charges repel while unlike charges attract
Michael Faraday and Joseph HenryBelieve that electricity is some kind of fluid.Michael Faraday – Discover theoretical basis of electricityJoseph Henry – Invents practical applications (eg: telegraph)
History of Electricity
Term ‘emf’ only used in electrical circuit. Term ‘pd- potential difference’ is used in all energy fields (eg: gravitational, electric etc)
EMF – electrical potential difference generated by a source (eg: generator and battery)
Potential difference can be measured between any two points
BUTEMF is measured between two ends of an energy source
Difference: EMF and pd
DEFINITION:The resistance of a conductor is the ratio of the pd applied across it, to the current passing through it
Resistance, R = pd across the conductor, V (Volt)
current through the conductor, I (Amps)
Resistance
V = Voltage (Unit: V)I = Current (Unit: A)R = Resistance (Unit: Ω
Ohm)
Equation : Ohm’s Law
V = IR
R = V/IOhm’s Law stated that:
Provided the temperature and other physical factors remain constant, the current through a wire is proportional to the potential difference across its ends.
Quiz30 volt lamp carries a current of 0.7 amperes. What is the resistance of the lamp?
R = V/I = 30/0.7 = 43 Ω
Obey Ohm’s Law V =
IR
1) More current flows2) Metal Filament gets hotter3) Filament gains energy, then the
atoms vibrate faster 4) Collision with the travelling
electron increases, thus resist their motion
Two resistances depends on its polarityWhich way it connected
Forward reaction = low resistanceReverse direction = High resistance
More current flow -> Temperature increases -> Thermistor makes available more free electrons to carry the current
Length (l)Cross-sectional area (A)
Material of which it is made
R = ρ /Aℓρ = resistivity (Unit: Ωm)
Resistance depends on :-
Length Long wire - more resistance Short wire – less resistance
Cross-sectional ->Thin wire- more resistance Thick wire- less resistance
Temperature -> in metals, a hot wire has more resistance than a cold wire.
POINTS!Metal has large number of free electronsWhen the electron move through the metal
lattice, electrons collide with the vibrating metal atoms
The collision oppose the flow of electrons. This causes metal to have resistance
Metal get heats up, thus metal atoms vibrate faster with higher amplitude. This causes electrons to become more difficult to pass through the lattice.
This increases the resistance of metal
Effect of Temperature on Resistance
Is the condition where: conductor is a perfect conductor -> completely
loses its resistivity Continue to carry current (persistent current)
when it’s kept below critical temperature, Tc.
The phenomenon occur in certain materials, usually at very low temperatures.
Occurs in a wide variety of materialsFor example, tin, aluminium and various metallic alloys
SUPERCONDUCTIVITY
For superconductor, the resistance drops abruptly to zero when material is cooled below its critical temperature.
Graph of superconductor
The transport Equation
I = current through conductorn = number of free electronsA = cross-sectional areav = drift velocitye = electron charge
From the information, we may deduce that:Volume of conductor = AℓNumber of free electrons - nAℓTotal charge on free electrons = Q = nA eℓ
I = Q/t * v, drift velocity of the electronsI = nA e / tℓ * 1 electron = 1.6 x 10 ^-19
C
v = /tℓ
I = nAve Transport Equation
Consider a piece of copper and a piece of silicon with
exactly the same cross-sectional area of 3 x 10^-6 m². If
both the copper and the silicon carry a current of 100µA,
what is the drift velocity of the electrons in each?
Assume that n (copper) is 10^29 m^-3 and n (silicon) is
2.6 x 10 ^18 m^-3
I = nAve v (copper) = (10^-4)
v = I/nAe (10^29) x (3 x 10^-6) x (1.6x10^-19)
= 2 x 10^-9 ms^-1
v (silicon) = (10^-4)
(2.6x10^18) x (3x10^-16) x (1.6 x 10^-19)
= 80 ms^-1
Quiz
Page 127 - Textbook
Potential Divider
V1 = IR1V2 = IR2
V1 = R1
V2 R2
Quiz
What is potentiometer?It uses the principle of potential divider circuit.
Potentiometer
Internal Resistance = Resistance to electric current within the source of emf.
Effects of internal resistance of a source of emf
Results in:- 1) Voltage across the terminals (potential
difference) of the source dropping as a current flows through it
2) Source being less than 100% efficient as energy is dissipated in the internal resistance as current flows through it.
Sources of emf – Internal Resistance
Formulae of Internal Resistance
Diagram – Battery with Internal Resistance
Graph: Power Against Load R